Boration Reaction

Following advances made in reduction reactions (vide supra), hydroboration and diboration have been the subject of intense investigation with NHC-Cu catalysts. Early work by Sadighi revealed that [(ICy)Cu(Ot-Bu)] efficiently catalyzed the 1,2-diboration of aldehydes. Mechanistic studies permitted to rationalize a number of features of this reaction and notably ruled out a possible oxidative addition pathway to favour c-activation of the diboron reagent by the copper centre. [(ICy)Cu(Ot-Bu)] was also used for the diaster-eoselective diboration—in fact, hydroboration after work-up—of sulfinyl aldimines.  

Finally, further developments of this methodology led to the report of a double hydroboration of terminal alkynes, yielding vicinal diboronates. The main features of this reaction are the use of a diboron reagent, instead of a borane, and its high chemo-, regio- and enantioselectivity (Equation (11.5)).  

Of note, Vicic et al. also observed an unexpected reaction of these CU-CF3 complex in solution—two molecules of [(SIMes)Cu(CF3)] being in equilibrium with [(SIMes)2Cu][Cu(CF3)2]—that will probably render mechanistic investigations more difficult.  

Finally, an oxidative coupling of 2-naphthols recently appeared using NHC-Cu species. While synthetically interesting heterocoupling between electronically differentiated naphthols was performed the nature of the NHC u species (coordination number, copper oxidation state) remains uncertain.  

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TRIMETHYLOXONIUM TETRAFLUORO-BORATE, 51, 142 Trimethyloxonium tetrafluoro-borate, reactions of, 51,... 

Abstract Transition metal catalyzed alkene and alkyne boration reactions are attractive... 

In view of the abundant experimental studies on the boration reactions catalyzed by transition metal complexes summarized above, theoretical studies to the detailed reaction mechanisms have also been carried out [25-28]. The main focus of these quantum chemical calculations has been to provide detailed structural and energetic information on the proposed reaction mechanisms. 

In this chapter, we will summarize the relevant theoretical studies on the reaction mechanisms of transition metal catalyzed borations. It is our hope that an overall picture can be given in a manner which can be easily understood without detailing all the theoretical aspects. It should be noted here that other comprehensive reviews, both experimental and theoretical, on the topic of catalytic boration reactions can also be found elsewhere in the literature [3-6]. 

In this chapter, theoretical studies on various transition metal catalyzed boration reactions have been summarized. The hydroboration of olefins catalyzed by the Wilkinson catalyst was studied most. The oxidative addition of borane to the Rh metal center is commonly believed to be the first step followed by the coordination of olefin. The extensive calculations on the experimentally proposed associative and dissociative reaction pathways do not yield a definitive conclusion on which pathway is preferred. Clearly, the reaction mechanism is a complicated one. It is believed that the properties of the substrate and the nature of ligands in the catalyst together with temperature and solvent affect the reaction pathways significantly. Early transition metal catalyzed hydroboration is believed to involve a G-bond metathesis process because of the difficulty in having an oxidative addition reaction due to less available metal d electrons. 

Trimethyl borate, reaction with Grignard reagents, 49, 91... 

Heating with metal oxides at elevated temperatures produces anhydrous borates. Reactions with halogens in the presence of carbon at temperatures above 500°C give boron trdialides. Heating a mixture of boric acid, ammonia and calcium phosphate in an electric furnace produces boron nitride. 

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